Wide band resonant directional couplers



May 10, 1960 P. G. MARIE 2,936,430

WIDE BAND RESONANT DIRECTIONAL'COUPLERS Filed June l5, 1956 2Sheets-Sheet 1 PIERRE G. MARNE May 10, 1960 v P. G. MARI WIDE BANDREsoNANT DIRECTIONAL couPLERs Filed June 15, 1956 2 Sheets-Sheet 2ATTORNEYS WIDE BAND RESONANT DIRECTIONAL COUPLERS Pierr G. Mari, Paris,France Application June 15, 1956, Serial No. 591,593

Claims priority, application France June 18, 1955 3 Claims. (Cl. S33-10)The present invention relates to wide band frequency selectivedirectional couplers for microwaves.

In applicants copending application Serial No. 416,869 led March 17,1954, there are disclosed frequency selective directional couplerscomprising two rectangular wave guides coupled to each other through acircular Wave guide stub which forms a resonant cavity. Such couplersmay be termed double biquadratic wave guide couplers. Each couplingbetween a rectangular wave guide and the circular wave guide stub is soestablished that a wave propagated down the rectangular guide inquestion and arriving at the vicinity of the coupling undergoes adivision of its energy into two parts. The first part is furtherpropagated Within the rectangular guide beyond the junction with thestub. The second part is radiated into the circular guide through asystem of coupling slots which are disclosed in that application. Theenergy thus passing into the circular guide appears there as a cr`cularly polarized wave comprising two TEU waves which are polarizedperpendicularly and which are in quadrature. The sense of rotation ofthis circularly polarized Wave depends upon the direction of propagationof the energy within the rectangular guide from which the energy passesinto the circular wave guide stub.

The resonant cavity provided by the circular wave guide stub can betuned to a selected frequency of resonance in order that the coupler maybe traversed only by energy lying within a band of frequencies centeredon the resonant frequency.

There are further described in the above-identified application groupsof couplers of the type just described but having one rectangular guidein common. These devices, which may be termed multichannel filters,hence include a common rectangular wave .guide adapted to support Wavescovering a Wide band of frequencies from which it is desired to extractseparately particular bands or channels of energy, and a plurality ofcircular wave guide stubs all coupled at one end to the commonrectangular wave guide, each of these stubs being coupled at its otherend to a separate rectangular guide. These circular Wave guide stubs actas cavity resonators and are tuned to the mid-frequencies of the severalchannels for which they are provided, and in each case the energy Withinsuch a channel or band may be obtained at one end of the separaterectangular wave guide coupled to such cavity. i

These directional couplers have a filtering or frequency selectiveproperty, the frequency selecting property being the resonance curve ofthe cavity. In the prior art, there were disclosed couplers having awide bandwidth which comprise two rectangular wave guides coupledtogether by means of a plurality of cavity resonators tuned to differentfrequencies lying within the band of frequencies to be transmittedthrough the coupler and spaced along the rectangular Wave guides atquarter wavelength intervals, the wavelength of reference being thecentral wavelength of niteci States Patent ICC the band. In thesecouplers of the prior art, each individual cavity resonator whichconveys a single mode wave does not form with the rectangular guides adirectional coupler. Directional coupling results merely from thespacing of the adjacent resonators at quarter wavelength intervals.Since the adjacent resonators are tuned to difierent frequencies, thereis not exactly an opposition of phase between two waves passing througha pair of such adjacent resonators and which are to be cancelled in agiven direction of the output rectangular wave guide. Neither is there aconcordance of phase between those two waves which are to be reinforcedin the other direction of the output rectangular wave guide.

It is an object of the present invention to provide frequency selectivedirectional couplers having a wide band width and having frequencyselection characteristics better than those of the resonancecharacteristics lof the cavities.

According to the invention directional couplers of the type disclosed inthe above-identified and in the present applications and having suitablyspaced resonant-frequencies are combined to provide plural-cavitycoupling and filtering units. However whereas in the prior art thecombination of plural directional couplers: was so made as to providefor a given frequency channel a single resonant cavity, according to thepresent invention there are provided for any given channel a pluralityof cavities which can be either cylindrical or annular, whose resonancefrequencies are graduated one after another, and whose number dependsupon the attenuation frequency characteristic desired for the filter.

The invention will now be further described in detail with reference tothe accompanying drawings in which;

Fig. l is a diagrammatic perspective View of a directional couplerhaving a single resonant cavity of the type disclosed in copendingapplication Serial No. 416,869;

Fig. 2 is a diagram useful in explaining the operation oflteringcouplers according to the invention incorporating couplers of thetype illustrated in Fig. 1;

Fig. 3 is a diagrammatic perspective view of a particular form offiltering directional coupler of the type of Fig. 2;

Fig. 4 is a graph describing certain properties of the coupler of Fig.3; and

Fig. 5 is a diagram of a switching filter including a plurality offrequency selective directional couplers of the type illustrated in Fig.3.

Referring to Fig. 1 which illustrates a directional coupler of the typedisclosed in my copending application above identified, referencecharacters 1 and 2 indicate two rectangular wave guides whereas 9 is acircular wave guide stub connecting the two rectangular guides togetherat their Wide faces, and 3 and 4 are systems of coupling slots betweenthe rectangular guides and the circular wave guide stub. The slotsystems may include each two perpendicular slots of dumbbell shape, thecenters of these slots being disposed on the axis of the circular guidealthough not on the axis of the rectangular guides. Rather these centersare displaced from the center line of the wide face of the rectangularguides by a quantity zo given by the relation:

Tan azo M in which a is the width dimension of the rectangular guide andkg is the wave length of the wave in that guide.

If to, ro, t1, r1, t2, r2 represent the intensities of the waves in theguides 1 and2 on opposite sides of the plane containing the axis of thecircular guide 9 and perpendic- 5 plication above identified that theseintensity magnitudes are related by the following matrix relation:

Here u is the impedance angle of the slots and is the half angle of thephase change of the wave in the circular guide.

The coeicent of transmission -r is given by t1- Sinz ue-ime-w T t0- 1Cosa ue-izoa-u) 2) and the coeficient reection p is given by 1'0 .2 sin(2H-u) cos ue-iZm-l 3 1 COS2 ue-iza-u) Equations 2 and 3 for r and pshow that if (Z0-u) =k1r k being an integer, then 1=1 for p=0 This meansthat all of the energy s transmitted.

If 0 is expressed as a function of the height h of a cavity thus:

wh s:

E 2g Y h (kJfW 2 The last expression makes it possible either todetermine the height h of the cavity in order to give to it a desiredresonant frequency f1 or else to calculate the resonant frequencies ofthe cavity for different values of k, lz being fixed.

.It is appropriate to consider how the coefficients of transmission 1-and reflection p vary Iin the vicinity of the resonant frequency f1.

Let d6 be the variation in 0 accompanying a variation from f1 of thefrequency of the waves propagated within the directional coupler. Inview of the expression for the wavelength in the guides one Vmay write@Lf-f1 (ny A being the wave length in free space.

If u is small, the intensities of the waves transmitted and reected varyrapidly about resonance and it is possible for a first approximation toreplace Equations 2 and 3 bythe following:

T=t 1= v l eY-fr4o2u (4) to f f1 1lJQ T l :tg: f1 fe-iua-m (5) Equations4 and 5 are obtained by replacing the functions of 0 by irst orderexpansions of d0 and by setting:

4K1r g 2 :Q1-tanz u k Equations 4, 5 and 6 make it possible to write thesquare of the matrix in the form:

Accordingly to the invention as schematically illustrated in Fig. 2 aplurality of cylindrical cavities of directional couplers I, P, M of thetype illustrated in Fig. l`connected in series, rectangular guides beingassumed to possess the length corresponding'to an electrical angle a.

If To, T1, TP l, Tp, Tm 1, Tm represent the intensities of the waves atthe entrance planes, at the planes of coupling of the couplers and atthe output planes of guide 1 and if T'o, T1, Tp 1, Tp, Tm 1, Tmrepresent the intensities of the waves at the planes of exit, ofcoupling for the successive couplers and at the exit from the guide 2,one may write By combining Equations 1, 7 and 8 one may obtain thefollowing matrix relation:

The matrix for the iilter of Fig. 6 then assumes the form The matrix (9)becomes much simplified in certain special cases.

Fig. 3 represents a filter in which p=2 and It includes the rectangularguides 1 and 2, the cylindrical cavity 9 vcoupled to the guides by thesystems of slots 3 and 4, and the cylindrical cavity 9 coupled to therectangular guides by slot systems 3' and 4.

The two rectangular guides are parallel, and the energy generated in thesecond rectangular guide is propagated thereinin a direction opposite tothe sense of propaga tion in the first guide. For a frequency f betweenthe frequencies f1 and f2, part of the energy is transmitted by thecavity 9 and the other part by the cavity 9'. The difference between thephase shifts undergone by these energies in rpassage throughthe circularguides is equal to `1r/2 .sinceone of the guides is detuned to lead 'andthe 5 other to lag. In the course of the passage through the rectangularguides the phase separation between the two parts of the energy amountsto 2a=1r. The total phase separation is hence 31r/ 2.

Matrix (9) then assumes the form f1 f 2 fl f2 If `the quality factorsare equal and if the frequencies f1 and f2 are close enough so thattheir arithmetical mean may be taken to be equal to their geometricmean, it is possible to write:

il o l T.' l

*f1-f2 fz-fi Q=Q1=Q2 fz-fi -Qfms If moreover T2 is assumed equal tozero, one may write as relations between the squares of the moduli forTo, T2 and TZ the following:

1 TOI2 Fig. 4 represents the variation of B as a function of thefrequency. In view of the approximations made in the above analysis, itis only the central portion of this curve which can be relied upon.

Fig. 5 represents a multichannel filter made up of frequency selectivedirectional couplers of the type shown in Fig. 3. The filter of Fig. 5makes it possible to extract from a wave of wide frequency make-upelectromagnetic energy fractions corresponding to three channels.

-The filter of Fig. 5 comprises a first frequency selective directionalcoupler including the rectangular guides 11 and 12 and the twocylindrical cavities 19 and 19. It also comprises a second frequencyselective directional coupler including the rectangular guides 21 and 22and the cylindrical cavities 29 and 29', and a third frequency selectivedirectional coupler including the rectangular guides 31 and 32 and thecylindrical cavities 39 and 39. The rectangular guides 11, 21 and 31 arealigned at their ends 17, 25 and 27, 35. Electromagnetic energy from anantenna not shown enters the coupler at the mouth of the guide 11.Energy appropriate to the first channel emerges at the mouth 16. Thatappropriate to the second channel emerges at the mouth 26 and thatappropriate to the third channel emerges at the mouth 36. The mouths 18,28 and 38 of the rectangular guides 12, 22 and 32 and also the mouth 37of the rectangular guide 31 are closed by dissipative load elements 13,23, 33 and 10.

1 claim:

1. A resonant directional wave guide coupler com- B= log db prisng aninput rectangular wave guide section open at both ends, an outputrectangular wave guide section of the same cross-sectional dimensionsparallel thereto, open at both ends, and a plurality of cylindricalcavity resonators having their axes perpendicular to the wide sides ofsaid rectangular guides, distributed lboth in space and resonantfrequency in such a way that the electrical angle in both rectangularguides between two successive cavity resonators is substantially equalto a quarter Wave length for a frequency between the resonantfrequencies of said successive circular resonators, each of saidcircular cavity resonators being coupled to said input and output waveguides by means of apertures centered about points at each of which thelongitudinal and transverse magnetic eld components are equal inamplitude and in phase quadrature, whereby to one sense of propagationin each of the rectangular guides corresponds a definite sense ofrotation of the wave in said circular cavity resonators, and a reversecoupling is eifected between said input and output rectangular waveguides.

2. A wide band resonant directional wave guide coupler comprising aninput wave guide of rectangular cross-section, said input wave guidebeing open at both ends and being adapted to support individualtraveling waves in both directions, an output wave guide having the samecross-section as said input wave guide and being disposed parallel tosaid input wave guide, a plurality of individual resonant frequencycavity resonators of circular cross-section adapted each to supportindividual circularly polarized waves in both senses of rotationinserted between said input and output wave guides adjacent of saidresonators being distant from one another by a distance substantiallyequal to a quarter wave length in said input and output wave guides fora frequency equal to the mean value of the resonant frequencies of saidadjacent resonators, and means to couple an individual circularlypolarized wave in each cavity to an individual traveling wave in bothinput and output wave guides.

3. A wide band resonant directional wave guide coupler according toclaim 2 in which said means comprise apertures in the wide faces of saidinput and output wave guides symmetrically disposed with respect to theaxis of References Cited in the le of this patent UNITED STATES PATENTSLewis May 6, 1952 Lewis Nov. 11, 1952 Pierce Jan. 27, 1953 FOREIGNPATENTS 592,224 Great Britain Sept. 11, 1947 Tillotson Aug. 17, 1954 A

